Strip transmission line microwave signal processing circuitry is commonly used in missile guidance and other similar applications requiring broad frequency coverage with functional versatility and economical manufacturing practices. The most significant physical phenomenon involved in strip line signal processing is the overtly imposed electrical interaction between two or more transmission lines or transmission line elements exhibited in the directional coupler.
The art strip line directional coupler design has evolved through the years as a result of the work of many contributors. In early days directional couplers were designed as interacting pairs of discrete transmission line sections (Cohn and others). Electrical length of the sections was set at one center frequency quarter wavelength. To broaden the frequency band width response it was necessary to implement the device as a sequence of such pairs with different mutual coupling coefficients. The practical problem of adapting the strip geometry to these requirements for progressively changing local coupling coefficients was methodically resolved through the use of a variable offset line geometry. Now, along with the push into frequency bands above 10 Ghz, has come the increasing incidence of severe degradation of the isolation and return loss operating characteristics. A major factor in causing the deterioration of performance with higher frequencies and broader band widths is found to be the requirement for high peak coupling coefficients coincident with an erratic distribution of currents laterally disposed on the offset line in the region near maximum coupling.